Coaxial electrode structure and a method of fabricating same



s. REINER 3,060,923

COAXIAL ELECTRODE STRUCTURE AND A METHOD OF FABRICATING SAME Oct. 30, 1962 Filed Jan. 7, 1959 INVENTOR STU/779T Fem/E7? ATTOR N EY York Filed Jan. 7, 1959, Ser. No. 785,993 7 Claims. (Cl. 128-21) This invention relates to coaxial electrode probing apparatus, and in particular, an improved coaxial needle electrode employed as a probing device and an improved method for making same.

The instant application is a continuation-in-part of the now abandoned Serial No. 697,986, filed November 21, 195-7, for Coaxial Electrode Structure and a Method of Fabricating Same.

Coaxial electrode needles currently in use particularly those employed in medicine and biology for sensing electrical activity of tissue are fraught with certain serious limitations. The electrode is essentially a long, thin, cylindrical current conducting needle provided with a centralized through bore through which an inner conductor wire is suspended. The inner conductor wire is generally coated with a varnish for the purpose of maintaining the inner conductor electrically insulated from the needle shaft of the electrode, which shaft serves as the electrode outer conductor.

As well known and appreciated by those skilled in the art, the electrodes currently in use have a relatively short life. The varnish or other insulating material coated on the centralized inner conductor breaks down in a relatively short time during use to cause electrical shorts. Of far more serious consequence, it is also well known that the prior art needle electrode cannot withstand the prescribed method of sterilization by means of autoclaving. The high temperature generated during autoclaving sterilization usually causes breakdown of the varnish insulation to short the needle conductors. Consequently, persons in the medical field often resort to the proscribed method of inserting the probing end of the needle in alcohol or some other liquid sterilizer to avoid insulation breakdown in order to prolong the life of the electrode. In addition, it is well known that the fabrication of prior art needle electrodes as practiced heretofore is not a simple procedure and is subject to a great deal of wastage.

It is, therefore, the principal object of the instant invention to provide an improved coaxial probing electrode and an improved method of making same wherein the problems attending fabrication of such electrodes heretofore experienced have been substantially eliminated particularly the problem of wastage. For example, by threading the electrode needle shaft on an inner conductor wrapped with fiber glass, it is found that by solidifying the fiber glass at the lead end of the sheathed wire, the needle may be easily threaded.

It is a further object of the instant invention to provide improved coaxial probing apparatus wherein the structure is sufiiciently strong by virtue of its novel features to withstand autoclaving sterilization so that the life of the electrode is rendered substantially infinite, that is to say, electrical shorts resulting from breakdown of insulation whether by reason of autoclaving or otherwise has been substantially eliminated.

It is a further object of the instant invention to provide a method for fabricating coaxial electrode probing apparatus which renders the manufacture of same economical and relatively simple and which permits the construction of a multi-inner conductor electrode with equal case.

Further objects and advantages will become apparent from the following description of the invention taken in conjunction with the figures, in which:

FIG. 1 is a longitudinal plan view, illustrating the technique of threading a plurality of electrode needle shafts on a fiber glass wrapped inner conductor and also illustrates a segment of an exposed portion of the sheathed wire dabbed with a non-toxic, inorganic resin in accordance with the practice of the instant invention;

FIG. 2 illustrates the coaxially mounted needle shaft depicted after same is bonded to the sheathed wire and wherein the Wire has been cut in preparation for the final steps of manufacture of the coaxial needle electrode;

FIG. 3 is a cross-section of the sheathed wire employed in accordance with the practice of the instant invention and is taken along line 33 of FIG. 1;

FIG. 4 is a cross-section of the same wire except that it illustrates the section thereof dabbed with bonding resin prior to drawing the needle shaft thereover and is taken along line 44 of FIG. 1;

FIG. 5 is a cross-section of the needle shaft centered over the sheathed wire prior to sliding same over the coated section depicted by FIG. 4 and is taken along line 5-5 of FIG. 1;

FIG. 6 is a cross-section of the needle electrode taken along line 6-6 of FIG. 2;

FIG. 7 is a longitudinal plan, exploded and sectional view, partly cut away, also showing the probing end of the apparatus suitably finished and is taken along line 7-7 of FIG. '6;

FIG. 8 is a longitudinal view in cross-section showing the elements comprising the electrical spliced end of the needle electrode prior to completed assembly thereof;

FIG. 9 is a longitudinal exploded sectional view, partly cut away, of the completed coaxial needle wherein the electrical spliced end of the needle shaft is shown in detail with insulating sleeves thereabout for securing that portion of the electrode needle against electrical shorts and for providing a mechanical handle surface;

FIG. 10 illustrates one method of bundling a sheathed wire in order to thread a needle shaft thereon for the purpose of fabricating an electrode having more than one inner conductor; and

FIG. 11 is a cross-section of the needle electrode taken along line 11-11 of FIG. 10 but with the bonding material interspersed between the inner conductors and bore surface of the needle shaft.

Reference is now made to the figures which illustrate the application of the aforesaid method and structural improvements with respect to a coaxial needle electrode adapted for use with an electromyograph amplifier designed for the purpose of sensing minute and rapidly changing neuromuscular potentials in the study of intrinsic electric activity of the nerves and skeletal muscles of a human body. However, it should be understood that the improvements are equally applicable to other types of probing apparatus of like characteristics.

Essentially the electrode 10 includes a needle shaft 11 of suitable longitudinal length. Needle shaft 11 may be made of stainless steel and has a centralized and cylindrical through bore 12. The improvements claimed herein are believed best illustrated by describing the fabrication of probing electrodes in accordance with the improved method. A plurality of needle shafts 11 are threaded on a sheathed wire 13 in the direction of arrow A. This is depicted in FIG. 1 wherein sheathed wire 13 is unraveled from its spool. In accordance with the teachings of the instant invention, sheathed wire 13 will consist of at least one layer of fiber glass 14 wrapped around a current conducting wire 15. Since wire 15 is needle bore 12.

trode 10. In the preferred embodiment, sheath 14 should include two layers of fiber glass wound one upon the other, spiral fashion, in opposite directions about inner conductor 15 in order to provide a sturdy, mechanically strong structural support for wire 15. This feature contributes to the elimination of breakage of said wire during fabrication of needle electrode and during its subsequent use as a probing instrument.

In accordance with the practice of the invention, the lead end of fiber glass wrap 14 is solidified, depicted as B in FIG. 1, by heating such end and then cooling same to transform the fiber glass at end B to a glass. When the fiber glass wrap is substantially solidfied at B, needles 11 are easily threaded on sheathed wire 13 as the lead end B is snaked through needle shafts 11, because the lead end B will not unravel or bunch. Thereafter, two or more needle shafts 11 are suitably threaded on sheathed wire 13. FIG. 1 shows two needles already threaded on sheathed wire 13 and the third about to be threaded thereon. Thereafter, the lefthand end of sheathed wire 13 may be cut, for example, at point C, as illustrated in FIG. 1. The insulating sheath 14 at the ends B, C may be cut back so as to expose wire 15 thereat. Sheathed wire 13, with the needles 11 mounted thereon, is then suspended in taut-like manner between two terminal posts, which posts are suitably connected to an electrical source. Mounted needle shafts 11 are then axially spaced apart one from another by a suitable distance to expose a section or segment of sheathed wire 13.

A non-toxic, organic resin 16 is dabbed on as many exposed portions of sheathed wire 13 as there are needles 11 mounted thereon. One of such resin coated exposed portions is depicted in FIG. 1 by the notation coating. Thereafter, each individual needle 11 is passed over a respective resin coated section of sheathed wire 13 by longitudinally displacing the needle shafts 11. Among other things, resin 16 is designed to bond needle shafts 11 to sheathed wire 13. Therefore it will be understood that each resin coated section of sheathed wire 13 will coextend axially at least the length of the needle drawn thereover. About this time, or just prior thereto, or soon thereafter, an electric current is caused to pass through the suspended sheathed wire 13 in order to heat same. It is perhaps preferable to start current flow through sheathed wire 13 soon after resin 16 is dabbed thereon and before sliding needles 11 over the corresponding resin coated portions because this will cause the coating of resin 16 to thin out along and flow into the fiber glass wrap 14 and thus facilitate impregnation of the fiber glass with resin 16.

In addition to the increased structural support supplied by fiber glass 14, this wrapping material also provides a uniform diametered sheath. The uniform diametered sheath inherently accommodates accurate centering of wire 15 with respect to needle shaft bore 12 atthe time needle 11 is drawn over the resin coated section of sheathed wire 13 such that during fabrication of electrode 10 and after resin 16 is cured, wire 15 is maintained substantially centered with respect to the coaxial needle shaft 11. Consequently, it will be understood that the inner conductor wire 15 is spaced from needle shaft 11 by a resin impregnated sheath 14 and resin 16 filling up the remaining concentric space between wrap 14 and This structural combination contributes measurably to eliminating electrical shorts particularly at the probing end 17 of electrode 10 after such probing end is machine finished as shown in FIG. 7 which as sures that the exposed end 18 of wire 15 is suitably spaced from the surrounding coaxial structure of needle shaft 11 at such probing end.

The heat generated by current flow through sheathed wire 13 commences curing of the dabbed resin 16 for the purpose of permanently bonding said sheathed wire to needle shaft 11. In summary, it will be understood that the use of fiber glass with the bonding resin 16, to be described in further detail hereinafter, provides certain structural advantages as well as facilitating fabrication of the electrode needle 10. The resin impregnated fiber glass in combination with the resin filler in the coaxial space between wrap 14 and needle shaft bore 12 improves the structural strength of the insulating medium which rigidly maintains inner conductor wire 15 within needle shaft 11 to increase longevity of the probing instrument as well as improving its insulating characteristics. And moreover, as noted hereinbefore, this combination maintains said inner conductor wire substantially centered within the needle bore.

Inasmuch as needle electrode 10 will be used to sense portions of a human body, it is important that a nontoxic resin is used for bonding and insulating purposes. To achieve this requirement, thermosetting resin 16 is made up of a mixture of an epoxy and polyamide, the latter serving as a catalyst or curing agent. It is, therefore, a further feature of the instant invention to employ as a bonding and insulating agent in combination with the foregoing needle electrode structure, a mixture made up of an epoxy resin, such as Epon 815, or Epon 820 or Epon 828, sold by The Shell Chemical Corporation, mixed with a polyamide resin such as Versarru'd #125 sold by The General Mills Corporation. The non-toxic, organic mixture 16 is composed of approximately 3 parts by weight of any one of the aforesaid epoxy resins with approximately 2 parts by weight of the aforesaid polyamide.

As noted hereinbefore, curing is initially commenced by passing a current through the suspended wire. T 0 ac celerate curing, it may be desirable to insert the suspended wire with the needles mounted thereon in an oven, such as infra-red oven, and to heat the same to C. During this accelerated heating process, current flow through wire 15 may be cut 01f. When the structures are heated for curing the resin either by current flow or by oven, it will be understood that the temperature of the structures is not raised to a value which would cause the fiber glass wrapping 14 to solidify and convert to glass.

After needles 11 are suitably bonded to sheathed inner conductor 13, the bonded structures are separated by cutting the wire extending from the needles at D and D depicted in FIG. 2. D, it will be understood, is at the probing end of needle 11. The sheathed wire 13 projecting from the other end of needle 11 is permitted to extend a suitable distance out from the needle, depicted as D on FIG. 7, so as to prevent a short between inner conductor Wire 15 and needle shaft 11. The sheath 14 at the extreme end of D will be cut back to permit an electrical splice to wire 15. It is also noted that needle shaft 11 has an enlarged portion 19 between its ends. A pair of twisted lead-in conductors 20, 21 are individually connected to the current conducting components of needle electrode 10 wherein one end of conductor 20 is soldered to the rear inclined wall of enlarged portion 19 and the correlated end of conductor 21 is soldered at 22 to the exposed extreme end of wire 15. The opposite ends of conductors are provided with well known plugs 23. Soldered connections are desirable because they provide low noise contact characteristics when probe 10 is used for electrical measurements.

The soldered connections of conductors 20, 21 to needle shaft 11 and wire 15 are protected against shorts and damage by a pair of hollow tubular insulating sleeves 25, 26 and insulating resin 24. Outer sleeve 26 may be made of hard rubber and serves as a handle. Sleeve 26 is inserted over the twisted wires 20, 21 and the rearward end of needle shaft 11. The forward end of sleeve 26 is passed over enlarged portion 19. The inner bore diameter of sleeve 26 is preferably slightly less than the outer diameter of enlarged portion 19 to effect a pressfit. The rearward end of sleeve 26 extends rearwardly beyond needle shaft 11 a suitable axial distance. The space within the bore of sleeve 26 is filled with insulating resin material 24. The aforementioned non-toxic organic resin 16 may be used for this purpose. In addition, some resin is dabbed over the front inclined wall of enlarged portion 19 at 27.

Smaller diarnetered sleeve 25 of suitable length is then passed over the twisted wires 20, 21 and the rearward end of needle shaft 11 until its forward end is substantially adjacent the rear inclined wall of enlarged portion 19. A clearance is provided to avoid pinching conductor 20 as it turns to connect with enlarged portion 19. In mounting sleeves 25, 26 over the connections of wires 20, 21 to the electrodes for the purpose of protecting same, it will be noted that outer sleeve 26 is first assembled on the needle shaft and inner sleeve 25 is then inserted into the outer sleeve bore filled with insulating resin material. This is desirable because upon inserting inner sleeve 25 into the space filled with potting material, air bubbles which may otherwise form therein are forced out. It will be understood, however, that the sleeves may be assembled in reverse order; but, the preferred sequence is that noted hereinbefore. Upon curing of bonding resin 24, sleeves 25, 26 and the electrode structure protected thereby are secured in relatively fixed position. Probing end 17 may then be machine finished by suitably grinding same in a manner well known in the art to provide a probing end of desirable shape.

Examination of probing end 17 in FIG. 7 will show that the slant edge of needle shaft 11 serves as the probing outer conductor wherein the exposed end 18 of wire 15 now held rigidly and substantially centered within its outer conductor by the aforesaid insulating combination of sheath 14 and bonding material 16, serves as the inner conductor, such that each is adapted to come in contact with a portion of the human body for measureelectrode structures, it will withstand autoclaving. A fur-' ther advantage of the aforesaid structure is that it allows construction of needle electrodes 10 of any desirable axial length.

The following dimensions are submitted merely to illustrate the sizes of certain portions of needle electrodes 10 employing the instant inventions. In one particular embodiment employing the aforesaid improvements, needle shaft 11 had a diameter of .01 inch for bore 12; the preferred diameter of wire 15 employed therewith is .004 inch wherein the wire is provided with a double layer of fiber glass which increased the overall diameter of the sheathed wire to .006 inch. Although a double wrapping of fiber glass is desirable as a preferred embodiment, a single layer of fiber glass may be used, for example, in the fabrication of a multi-inner conductor coaxial electrode next to be described, the sheathed wire to be employed therewith includes a single wrapping of fiber glass.

It will also be understood that the present invention is equally applicable to multiple inner conductor coaxial needle electrodes wherein each electrode is provided with two or more insulated inner conductors within needle bore 12. FIGS. 10 and 11 show the initial steps of constructing a two-wire inner conductor electrode wherein a suitably long sheathed wire 13 is cut from its spool and folded over as shown in FIG. 10. The ends are alined and bunched and the tip ends 30 are then heated and 6 cooled in order to solidify the fiber glass as a unitary glass structure to facilitate threading of needle shaft 10 thereon. After needle shaft is threaded on the folded sheathed wire, the wire at the back end where it is looped may be out and spliced together for electrical contact at one terminal of a source whereas the front ends may be cut and exposed for conductive connection to the other terminal of an electrical source. In all other respects, the needle electrode is made in substantially the same way as described hereinabove, that is to say, a non-toxic resin 16 is dahbed on the wires held taut by suitable suspension between the terminal supports. Needle 11 is moved over the resin covered segment. The resin is cured for bonding the needle to the wires. It has been found that the resin impregnated fiber glass wires are suitably spaced apart through the length of the needle bore to preclude shorts.

Since the foregoing described electrodes are designed to withstand autoclaving, it will be understood that lead-in conductors to be connected thereto also should be characterized to withstand autoclaving.

It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Electrical probing apparatus comprising, an electrical conducting member having a through bore, an electrical conducting wire extending through said bore, a substantially uniform diameter sheath of fiber glass wrapped around said wire, and a non-toxic resin impregnating said sheath and interspersed between said sheathed wire and the surface of said through bore for securing said sheathed wire to said member, said wire being suitably spaced within said bore by said sheath and said bonding resin to prevent an electrical short, said member having a frontal probing end and a rearward end and an intermediate enlarged diametrical portion, said sheath wire extending outwardly from the rearward end of said probe member and having a bared portion for electrical connection thereto, lead-in conductors individually connected to said member and bared wire portion, a sleeve of electrical insulating material mounted over the rearward portion of said member and the connection to said bared wire portion, a second sleeve of electrical insulating material extending around said first sleeve and engaging said enlarged portion, and means filling the spaces between said sleeves and member and for bonding the aforesaid elements in relatively fixed position.

2. Apparatus as defined in claim 1 wherein said nontoxic resin comprising, about 3 parts by weight of an epoxy and about 2 parts by weight of a polyamide.

3. A coaxial electrode comprising, an electrical conducting needle shaft having a through bore, an electrical conducting wire extending through said bore, a sheath of fiber glass wrapped around said wire, and a non-toxic resin coating impregnating said fiber glass and bonding said sheathed wire within said needle shaft, said resin filling the space between said sheath wire and the bore of said needle shaft, said sheathed wire being substantially centered therein, said needle shaft having a frontal probing end and a rearward end, the probing end of said shaft being suitably shaped to permit the object to be probed to come into electrical contact with the exposed end of said wire and with the coaxially surrounding needle shaft, said wire projecting outwardly from the rearward end of said shaft and having a bared portion, lead-in conductors individually connected to said bared portion of said wire and said shaft, an insulating sleeve extending over a substantial portion of said shaft from its rearward end, and insulating material filling the space between said sleeve and shaft and for bonding sleeve and shaft in relatively fixed relationship.

4. Apparatus as defined in claim 3 wherein, said resin comprising, a thermosetting resin of about 3 parts by weight of an epoxy and about 2 parts by weight of a polyamide.

. 7 a 5. A coaxial needle electrode comprising, an electrical conducting needle shaft having a through bore, a plurality of electrical conducting wires extending through said needle bore and each wire having at least a single layer of fiber glass wrapping, a non-toxic resin impregnating said ,wire wrapping and interspersed between the sheathed lwires an d bore surfacejof said needle shaft and thus filling the spacing therebetween and bonding the inner conduca tors to said needle shaft, the probing end of said coaxial :electrode bein g suitably shaped to permit the object to be probed to come into electrical contact with the spaced apartfends of said wires exposed thereat and with the ,coaxially surrounding probing end of said needle shaft,

said wires having bared portions at the rearward end of said shaft, lead-in conductors providing individual electrical connections to each of said bared wire portions and :to said shaft, and insulating means surrounding said electrical connections for protecting same and extending over a substantial portion of said shaft from its rearward end for permitting manual holding of said electrode.

6. Electric probe apparatus comprising, an electrical conducting member having a probe end and a rearward end and also having an internal axial bore, an electrical conducting wire extending along said member bore, a

.shcathof fiber glass wrapped around said wire, electrical insulating and non-toxic resin impregnated in said wire sheath and between same and said member for .bonding said sheathed wireinsaidmember, said wire being electrically insulated from ,said ,member by said sheath and resin, said wire having a bared portion at the rearward-end of said member, lead-in conductors providing vindividualelectrical connections-to said bared wire portion and said member, and insulating means surrounding said electrical connections for protecting same and extending over a substantial portion of said member from its rearward end for permitting manual holding of the probe.

7. Apparatus as defined in claim 6 wherein said nontoxic resin comprising, about 3 parts by weight of an epoxy and about 2 parts by weight of a polyamide.

References Cited in the file of this patent UNITED STATES PATENTS 

